How can i select by number of vertices?
Sounds like the script you downloaded has a bug or is used differently.
You can try contacting the author and describe the error, s/he might fix it.

Anyway, you can paste this to your text editor, change the value of vertCount and press "Run script", it will select all the faces with the number of verts specified in the vertCount variable.

import bpy, bmesh

vertCount = 3 # &lt;-- Faces with this amount of verts will be selected

bpy.ops.object.mode_set( mode = 'EDIT' )
bm = bmesh.from_edit_mesh( bpy.context.object.data )

bpy.ops.mesh.select_mode( type = 'FACE' )
bpy.ops.mesh.select_all( action = 'DESELECT' )

for f in bm.faces: 
    if len( f.verts ) == vertCount: 
        f.select = True

bm.select_flush(True)